Dewaxing is a process for treating petroleum fractions so as to remove certain hydrocarbons, particularly straight chain and slightly branched chain paraffins, that solidify readily (waxes). Dewaxing can be effected by solvent extraction and crystallization but in recent years attention has focused on processes that remove waxes by selective catalytic conversion of the straight and slightly branched chain paraffins. An example of such a process is disclosed in U.S. Reissue Pat. No. 28,398, in which the dewaxing catalyst is the molecular sieve ZSM-5.
In the case of lubricating oil basestocks, catalytic dewaxing is often accompanied by a decrease in viscosity index (VI) which is undesirable since VI is a measure of the ability of the oil to retain its viscosity with increasing temperature. Catalytic dewaxing generally functions via two mechanisms, isomerization and cracking, and the loss in VI is typically associated with the cracking mechanism. There is therefore interest in developing dewaxing catalyst systems that show high selectivity towards isomerization and low selectivity towards cracking.
For example, U.S. Pat. No. 4,222,855 discloses that low pour point lubricating oils with a high VI can be produced from waxy hydrocarbon fractions boiling within the approximate range of 450 to 1050° F. (232 to 566° C.) by contacting the waxy fractions with a catalyst comprising an aluminosilicate zeolite selected from ZSM-23 and ZSM-35 and a hydrogenation metal. The resultant dewaxed oil is said to have a V.I. considerably higher than that obtained with a ZSM-5 catalyst.
In addition, U.S. Pat. No. 5,075,269 discloses that waxy hydrocarbon oils can be catalytically dewaxed to produce high viscosity index lubricating oil stocks utilizing as a catalyst the acidic zeolite ZSM-48 and in particular ZSM-48 synthesized using a organic linear diquaternary ammonium compound as the structure directing agent. Again, the dewaxed oil is said to have a higher V.I. than that obtained with ZSM-5.
A high aluminum form of ZSM-48, having a silica:alumina molar ratio of 110 or less and being free of non-ZSM-48 seed crystals and free of ZSM-50, is disclosed in U.S. Published Patent Application No. 2007/0131581 and is shown to exhibit enhanced activity as a dewaxing catalyst as compared with conventional forms of ZSM-48, such as that employed in U.S. Pat. No. 5,075,269.
It is also known to effect dewaxing of hydrocarbon feedstocks using a pluarlity of different zeolitic materials, arranged sequentially or combined in a single catalyst bed, in an attempt to improve the properties of the final dewaxed product.
For example, U.S. Pat. No. 4,599,162 discloses a cascade catalytic dewaxing process comprising a) passing a hydrocarbon feedstock containing long chain normal paraffins and long chain slightly branched paraffins, over a first crystalline silicate zeolite selected from ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48, and/or TMA Offretite, preferably ZSM-12, the first zeolite having catalytically effective amounts of a hydrogenation/dehydrogenation component in the presence of hydrogen at a temperature between 450 and 700° F. (232 and 371° C.), a pressure of about 400 psig, a hydrogen feed rate of about 2500 SCF H2/bbl, and a LHSV between 0.2 and 6.0; and (b) passing the entire effluent from step (a) over a second crystalline silicate zeolite being different from the first zeolite and selected from ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38, ZSM-48, TMA Offretite, Clinoptilolite, and/or Erionite, preferably ZSM-5, the second zeolite having catalytically effective amounts of a hydrogenation/dehydrogenation component in the presence of hydrogen at a temperature between 500 and 700° F. (260 and 371° C.), a pressure of about 400 psig, a hydrogen feed rate of about 2500 SFC H2/bbl, and a LHSV between 0.2 and 2.0. The cascade relationship of the different medium pore zeolites is said to offer superior dewaxing activities and lube yield, higher V.I., improved catalyst stability in the second stage and flexibility in catalyst regeneration as compared with prior art dewaxing catalyst systems.
In U.S. Pat. No. 6,051,129 there is a disclosed a process for reducing the haze point of a hydrocarbon oil feedstock, which has a major portion boiling over 1000° F. (538° C.), in which the feedstock is contacted with a catalyst system comprising Zeolite EU-1 and a catalyst selected from the group consisting of SSZ-32, ZSM-48, and mixtures thereof.
Moreover, U.S. Published Patent Application No. 2007/0029229 discloses a dewaxing process comprising contacting a hydrocarbon feedstock under dewaxing conditions with a catalyst comprising a combination of zeolites having the MTT framwork type, such as ZSM-23, and the GON framework type, a 12 ring/8 ring zeolite with uni-dimensional channels, such as GUS-1. When used to dewax a hydrocarbon oil feedstock having a major portion boiling over 1000° F. (538° C.), the specific zeolite combination is said to significantly increase the conversion of heavy wax (long n-alkanes), thereby lowering the cloud point of the product.
U.S. Published Patent Application No. 2007/0029230 discloses a dewaxing process comprising contacting a hydrocarbon feedstock under dewaxing conditions with a catalyst comprising a combination of zeolites having the MTT framwork type, such as ZSM-23, and the MTW framework type, such as ZSM-12, the MTT and MTW zeolites having a crystal size less than 0.1 micron. When used to dewax a hydrocarbonaceous feed, the MTT/MTW zeolite combination is said to improve the viscosity index of the dewaxed product as compared to that of the waxy feed.
Despite these advances there remains a need for improved dewaxing catalyst systems and processes, particularly which exhibit a combination of high activity and selectivity for the isomerization of n-paraffins in lubricating oil basestocks so as to produce high VI lubes in high yield.
In accordance with the present invention, it has now been found that a dewaxing catalyst system and process that employs a combination of ZSM-48 and an MTT framework type material, such as ZSM-23, exhibits such a desirable combination of isomerization activity and selectivity, particularly when both molecular sieves are in high aluminum forms.